diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index c0815198c20685d2e29e6803715224253c331390..5a1419415b6b88295c8f2151109038445a3285e0 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Add deprecation warning for inferred foreign key. #6029 [Josh Susser]
+
 * Fixed the Ruby/MySQL adapter we ship with Active Record to work with the new authentication handshake that was introduced in MySQL 4.1, along with the other protocol changes made at that time #5723 [jimw@mysql.com]
 
 * Deprecation: use :dependent => :delete_all rather than :exclusively_dependent => true.  #6024 [Josh Susser]
diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 0771f262182834c6f887e594f9de6fc3add987d0..33fd8afee239dd17aec2eb6a4f5ac63f4adeb9ae 100755
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -655,6 +655,12 @@ def has_one(association_id, options = {})
       #              :conditions => 'discounts > #{payments_count}'
       #   belongs_to :attachable, :polymorphic => true
       def belongs_to(association_id, options = {})
+        if options.include?(:class_name) && !options.include?(:foreign_key)
+          ::ActiveSupport::Deprecation.warn(
+          "The inferred foreign_key name will change in Rails 2.0 to use the association name instead of its class name when they differ.  When using :class_name in belongs_to, use the :foreign_key option to explicitly set the key name to avoid problems in the transition.",
+          caller)
+        end
+        
         reflection = create_belongs_to_reflection(association_id, options)
         
         if reflection.options[:polymorphic]
diff --git a/activerecord/test/associations_test.rb b/activerecord/test/associations_test.rb
index b6ffda81059b6e8204cc3701a552295b8b9a2349..d7997beaa316af5165ed804615e06d2778e4e64b 100755
--- a/activerecord/test/associations_test.rb
+++ b/activerecord/test/associations_test.rb
@@ -350,6 +350,13 @@ def test_save_still_works_after_accessing_nil_has_one
     end    
   end
   
+  def test_deprecated_inferred_foreign_key
+    assert_not_deprecated { Company.belongs_to :firm }
+    assert_not_deprecated { Company.belongs_to :client, :foreign_key => "firm_id" }
+    assert_not_deprecated { Company.belongs_to :firm, :class_name => "Firm", :foreign_key => "client_of" }
+    assert_deprecated("inferred foreign_key name") { Company.belongs_to :client, :class_name => "Firm" }
+  end
+
 end